Response smuggling via wrapped FCGI record length
Published Jun 18, 2026 · Updated Jun 18, 2026
Integer overflow in HAProxy 3.4.0 and earlier allows remote attackers controlling a FastCGI backend to desynchronize response parsing. In src/mux_fcgi.c, the ignore-record path adds the padding length to the 16-bit fcgi_conn.drl field, so 65,535 plus any padding wraps to zero and leaves the record bytes unconsumed. Exploitation requires a backend that can return a crafted FastCGI record; the leftover bytes are then parsed as headers, enabling response smuggling and routing errors.
Summary
What happened
Integer overflow in HAProxy 3.4.0 and earlier allows remote attackers controlling a FastCGI backend to desynchronize response parsing. In src/mux_fcgi.c, the ignore-record path adds the padding length to the 16-bit fcgi_conn.drl field, so 65,535 plus any padding wraps to zero and leaves the record bytes unconsumed. Exploitation requires a backend that can return a crafted FastCGI record; the leftover bytes are then parsed as headers, enabling response smuggling and routing errors.
The record
- CVE
- CVE-2026-55203
- Published
- Jun 18, 2026
- Updated
- Jun 18, 2026
- Vendor
- HAProxy Technologies
- Product
- HAProxy
- Classifications
- CWE-190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jun 18, 2026CVE publishedPublication date reported by the CVE source.
- Jun 18, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
Compare your product and version with the public record. A matching version still requires validation against your environment.
Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=0 <=3.4.0
What conditions does exploitation require?
What is affected?
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
No public exploit references are available in this record.
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
See the directory against your own environment.
Your stack
Check the software in your environment
Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.
Book a demo